Thermionic valves



Dec. 12, 1961 A. G. FIELD 3,013,176

THERMIONIC VALVES Filed Feb. 5, 1960 FIGJR;

' Inventor A.G.FIEI.D

Attorney United States PatentfO Filed Feb. 3, 19 60-,.Ser;-No."6,537Claims priority, application Great Britain Feb. 17,1959 8 Claims. (Cl.313-350) The present invention relates to grids for thermionic valves. v

The control grid-and also in some instances, other grids-of a thermionicvalve is nowadays often formed of exceedingly fine tungsten. wire. In'what is ,known as a frame grid this wire is wound around a pair of siderods, spaced apart and held to make a rigid frame by a pair of strapsbetween which the winding of fine wire is laid down. In other types ofvalve a planar grid is used in which the wire is stretched across oneface of arectangular'or annular frame.

With a view to increasing, more and-more, the ratio of mutualconductance to cathode-grid capacitance in a thermionic valve, the gridwire is made as fine as possible and wire between S and 10 microns indiameter has already been used (one micrn10- meter). Performance couldbe still further improved if yet finer wire could be used.

According to the present invention the grid-wire in a grid for athermionic valve is provided by gold coated quartz fibre. Such fibrescan be produced in diameters of 2 to 3 microns and of adequate tensilestrength.

The invention will be described with reference to the accompanyingdrawings in which:

FIG. 1 shows diagrammatically a preferred method of producing therequired lengths of quartz fibre from a solid rock of quartz;

FIG. 2 illustrates a frame grid according to the invention on a muchenlarged scale;

FIG. 3 illustrates a single-sided planar grid; and

FIG. 4 illustrates a preferred method of gold-coating the quartz fibre.

One known method of producing very fine quartz fibre, which has been inuse now for many years, is to heat the end of a quartz rod to near itsfusion point to draw off a fibre and attach it to the head of an arrowfitted in a cross-bow and to fire the bow, drawing off a length offibre. In this way skilled technicians can obtain lengths ofapproximately to 20 feet of fibre of the required 2 to 3 micronsdiameter; such lengths are of the order of that required for Winding aframe grid. A preferred method of manufacture, however, is illustratedin FIG. 1 in which a rod of fused quartz 1 is driven slowly along itslength through a set of oxy-gas jets 2, which melt the end of the rod. Afibre 3 is drawn off from the molten end and passed round a revolvingwheel 4 of large diameter. For the equilibrium condition in which thewheel 4 picks up just as much fibre in unit time as is being molten fromthe rod 1, consideration of the volume of fibre being wound on the wheelper unit time with the corresponding volume of molten rod, shows thatthe diameter of the fibre can be controlled by adjustment of therelative speed of rotation of the wheel 4 with respect to the axialmovement of the rod 1. Thus, for a rod of 0.040" diameter a wheel 6 feetin diameter rotating at 200 revolutions per minute would draw olf afibre 2 microns in diameter when the rod is moved forward at 0.0175 inchper minute.

The fibre taken off by the wheel 4 or obtained in other manner is woundon to small spools and is goldcoated either before or after windingon.the grid sup port. For the gold-coating known sputtering techniques maybe used or, as is'at present preferred and illustrated in FIG. 4, wherethe coating is to be applied before winding, the fibre 10 is unwoundfrom a'srnall diameter spool l'land run through a gold helix 1 2heatedin an oven 13 by means not shown, so that the gold is 'de positedevenly 7, over the whole of the-surface. The gold coated wire is rewoundonto a second spool 14. The whole is. enclosed in a vacuum. chamber 15.

For a frame grid the gold-coated quartz fibre is wound on toa'standardgold-plated grid. framewith the normal manufacturingequipment. the spools are rotated round the frame, their rotationbeingcontrolled either by a frictional pulley or by a torque motor. The

torque produced must be adjusted to provide -a tension in the fibre*ofless than 0.2 gramme. Oncompletion of the winding, the grid is firedat 1l50. C. ina reducing atmosphere to braze together the gold on-thefibre to the gold on the frame; the grid is then ready for use.-Alternatively the fibres can 'beadhered to the frame by 'using a glassfrit and ovening at 800 C. in hydrogen.

A completed frame .grid according to the invention is illustrated inFIG. 2, in which side rods are indicated at 5 spaced apart by straps 6,welded to the side rods and the grid wire 7 is wound over the side rod 5between the straps 6. Typical dimensions for a grid are:

Diameter of side rods .0345 inch. Distance apart of side rods .250 inch.Width of straps 6 .020 inch. Thickness of straps 6 .005 inch. Distanceapart between straps 6 .250 inch. Length of winding between straps .250inch i.e. space between straps completely wound.

A typical grid according to the invention uses 2 micron Wires wound at1000 turns per inch, which thus provides a very uniform electricfieldwith a minimum interception factor for the electrons. The penetration ofthe field between the wires to the cathode surface is adequatelycontrolled if the pitch is not greater than the distance from cathode togrid. With a finer wire the same shadowing effect takes place at asmaller pitch therefore regaining control over anode current at the samegrid to cathode capacitance. Thus the ratio of material conductance togrid-cathode capacitance is improved.

In a normal frame grid the straps 6 are made of molybdenum and arewelded to the molybdenum side rods. In the present invention, the straps6 must be made of a material whose coefficient of thermal expansion isnot too different from that of the quartz fibre e.g. it can be made ofquartz strip which will unite readily to the molybdenum side rods andhold the frame sufiiciently rigid to withstand the tension of thewinding and maintain it in position during use. Alternatively, the wholeof the frame may be made of quartz or of a refractory ceramic having atemperature coefficient of expansion approximating that of quartz inwhich case difficulties due to differential thermal expansion do notarise.

A planar grid is illustrated in FIG. 3. Here a ring of fused quartz 8 isused as a support and quartz fibres are stretched over the ring andsecured thereto by means, for example, of a glass frit or the fusion ofgold coatings applied before the winding operation. The grid, exceptthat it is of quartz, which must, at some stage, be gold- Patented Dec.-12,1961

coated, is similar in construction and manufacture to known metal grids.In particular, if the support 8 is rectangular, the quartz fibre formingthe grid wire could conveniently be wound over two ends of the support,as in the frame grid embodiment described above, and the winding cutaway from one face after being secured to the support.

At the present time the material preferred for the nonmetallic portionsof FIG. 2 and the support 8 of FIG. 3

is'quartz. Refractory ceramics of very low temperature cvoefficient ofexpansion are, however, becoming available and it is envisaged that itwill soon be practical, and even preferable, to use such ceramics forthe straps or support, respectively.

While the principles of the invention have been described above inconnection with specific embodiments, and particular modificationsthereof, it is to be clearly understood that this description is madeonly by way of example and not as a limitation on the scope of theinvention.

What is claimed is:

1. A grid for a thermionic valve in which the grid wires are ofgold-plated quartz fibre.

2. A grid for a thermionic valve comprising a pair of side rods, a pairof transverse straps spaced apart and joining the side rods to form arigid frame, and a winding,

over the side rods and between the straps, of gold-coated quartz fibre.

3. A grid according to claim 2 in which the said frame is of quartz.

4. A grid according to claim 2 in which the said frame is of arefractory ceramichaving a temperature coefficient of expansionapproximating that of quartz.

5. A grid according to claim 2 in which the said side rods are metallicand the straps are of quartz strip united to the metal of the side rods.

6. A grid according to claim 2 in which the side rods are gold-platedand the winding is bonded to the side rods by the fusion of the gold onthe quartz fibre and on the side rods.

7. A planar grid comprising a frame support of quartz to which aresecured wires of gold-coated quartz fibre.

8. A planar grid comprising a frame support made of a refractory ceramichaving a temperature coefficient of expansion approximating that ofquartz, to which are secured wires of gold-coated quartz fibre.

Walsh Dec. 1, 1953 Kerstetter June 2, 1959

